As a novel energy characterized by cleanness, reproducibility and enormous potential, wind power is rapidly developed throughout the world in the aspect of power generation. Wind power generator system based on the doubly fed induction generator becomes commercially dominant. Shown as FIG. 1, a stator side of a generator 10 is directly connected with a power grid 20 and a rotor side is connected with the power grid via converters. In general, the converter can be divided into a grid-side converter 60 connected with the power grid and a generator-side converter 40 connected with a du/dt inductance 80 which is connected to a generator rotor, and the generator-side converter 40 is connected with the grid-side converter 60 via a direct current bus (and a direct current bus capacitance 50). The converters 40 and 60 are controlled by a controller 70. The controller 70 controls the current at the rotor side of the generator via the converters in order to control active power and reactive power of the generator.
As the installation capacity increases continuously, the proportion of the power generation capacity of the wind power generator system in the power grid gets higher and higher. In the event that the dip fault of the power grid occurs, the wind power generator system is disengaged from the power grid and comes to a halt so as not to provide frequency and voltage support for the power grid, thereby disfavoring safe operation of the power grid extremely. Therefore, nations all over the world successively come up with a series of operating standards for the wind power generator system, including the power grid fault ride-through ability, i.e., when instant dip of the power grid occurs, the wind power generator system is required to maintain the grid-connected state, and provides certain reactive power support during the fault in order to help recover the power grid as much as possible.
Since stator power is indirectly controlled by controlling rotor current and the doubly fed induction generator the stator of which is directly connected to the power grid is directly impacted by the power grid, the wind power generator system based on the doubly fed induction generator is quite sensitive to disturbance of the power grid so that the system, during the power grid fault, is harder in control, and even out of control. In case of the instant dip of the power grid, demagnetizing process is formed in the generator, and this process leads to rapid rise of the stator current and the rotor current. In case of the large dip of the power grid, overcurrent occurs in the stator and the rotor, and the converters will be destroyed if additional measures are not adopted.
In order to solve the above problem, the technical way that a passive crowbar circuit is arranged is adopted in the prior art. Shown as FIG. 1, in general, a passive crowbar circuit 30 is connected in parallel to the front end of the du/dt inductance 80 (i.e. between the du/dt inductance 80 and a rotor winding of the doubly fed induction generator 10).
Shown as FIG. 2, the passive crowbar circuit 30 is composed of a three-phase uncontrolled rectifier bridge, which is composed of diodes 31, and an energy consumption resistance 32 and a thyristor 33 or other half-controlled power electronic devices, which are serially connected to a direct current side of the rectifier bridge. In case of the instant dip of the power grid, the thyristor 33 in the passive crowbar circuit is triggered to achieve conduction and the generator rotor is short-circuited so that the converters 40 and 60 are disengaged from the grid and come to a halt fast, thereby achieving the purpose of protecting the converters.
However, the half-controlled power electronic devices adopted by the passive crowbar circuit are subject to conduction upon the power grid power fault, and after the converters are protected, cannot be switched off before the energy of the generator rotor winding is depleted, thus the half-controlled power electronic devices cannot be switched off before the converters are disengaged from the power grid and the residual energy of the generator rotor winding is depleted, in this case, the power grid fault power occurs, and the generator cannot operate continuously, namely, the generator does not include the ability of power grid fault ride-through.
In order to solve the problem, an active crowbar circuit is provided. What is shown as FIG. 3 is the active crowbar circuit used commonly at present, the active crowbar circuit is composed of the three-phase uncontrolled rectifier bridge, which is composed of diodes 31, and the energy consumption resistance 32 and a full-controlled power electronic device 34, which are serially connected to the direct current side of the rectifier bridge. In case of voltage dip fault of the power grid, the controller 70 switches off the generator-side converter 40, the active crowbar circuit is simultaneously triggered to achieve conduction, and the generator rotor is short-circuited, thereby protecting the generator-side converter 40. In case that instantaneous energy generated by the power grid power fault attenuates to a set value, the controller 70 switches off the active crowbar circuit and the generator-side converter 40 is simultaneously switched on to provide reactive power support for the power grid 20 and help recover the power grid fast, thereby achieving power grid fault ride-through.
The active crowbar circuit solves the problems of the passive crowbar circuit, but the following defects are still present:                1. when the active crowbar circuit is connected between the du/dt inductance 80 and the rotor winding of the doubly fed induction generator 10, shown as FIG. 1, the current, which flows through the crowbar circuit and the generator rotor winding formerly, continuously flows via loops of the du/dt inductance and the generator-side converter at the moment the active crowbar circuit is switched off under the control of the controller 70; no current flows inside the du/dt inductance 80 during the conduction of the crowbar circuit, and when the crowbar circuit is switched off, the current flowing through the du/dt inductance 80 increases instantaneously to generate quite high terminal voltage at two ends of the du/dt inductance 80, so that a high voltage is also generated at an input end of the crowbar circuit connected with the du/dt inductance 80, and if the high voltage cannot be eliminated timely, it may break through the full-controlled power electronic device 34 at the direct current side of the crowbar circuit, reducing the operating reliability of the active crowbar circuit.        2. when the active crowbar circuit is connected between the du/dt inductance 80 and the generator-side converter 40, shown as FIG. 4, such a connection way can relieve the high voltage generated at the input end of the crowbar circuit during the switch-off of the crowbar circuit, however, an output end of the generator-side converter 40 outputs a voltage pulse with high frequency and high amplitude during normal operation of the generator system, and as the reverse recovery time of the diodes 31 forming the uncontrolled rectifier bridge in the active crowbar circuit is long in general, the voltage pulse with high frequency and high amplitude acts on the diodes 31 lastingly to lead the diodes to heat accumulation, and the service life of the diodes 31 is greatly shortened in case of long-term operation, thus the service life of the active crowbar circuit is tremendously limited.        